Serial high-speed interfaces use typically one differential wire pair or one optical fiber, also called one lane, per direction. For instance, two pairs are used for bidirectional transmission, in data transmission as, for instance, in serial ATA (advanced technology attachment) and Peripheral Component Interconnect (PCI) Express (PCIe), which uses existing PCI programming concepts, but on a completely different and much faster serial physical-layer communications protocol.
In PCIe, at the electrical level, each lane utilizes two unidirectional low voltage differential signaling pairs at 2.5 Gb/s. Transmit and receive are separate differential pairs, for a total of 4 data wires per lane. As with all high-speed serial transmission protocols, clocking information must be embedded in the signal. At the physical level, PCIe utilizes the very common 8B/10B encoding scheme to ensure that strings of consecutive ones or consecutive zeros are limited in length, so that the receiver does not lose track of where the bit edges are. This coding scheme replaces 8 uncoded (payload) bits of data with 10 (encoded) bits of transmitted data, consuming 20% of the overall electrical bandwidth.
Alternatively, one lane may be used for both directions, by using of the lane with a time division access (time-shared) scheme, where given (or negotiated) time slots are used for transmission from end A to end B or vice versa, respectively, which is used in Universal serial Bus (USB), for example.
Even simultaneous bidirectional transmission is a very old method in telecommunication; one problem in conventional systems is formed by the signal reflected back to the transmitting unit. There is no method for eliminating this reflected part without undue efforts as besides other very complicated calibration to every cable length and transceiver return loss behavior.
The basic principle of simultaneous bidirectional signaling is illustrated in FIG. 1. If unit A sends a pulse via cable 10 to unit B, part of the pulse reflects back to unit A from impedance discontinuities in connectors 21, 22, and impedance mismatch of the intended receiver 32 (and/or electro static discharge (ESD) suppression components (not shown)) at unit B. This results in only low bit rate signaling in simultaneous bidirectional links having connectors, as it is the case in all of the above-discussed applications.
Examples for and further information on high-speed links can be found, for instance, in “A 2.4 Gb/s/pin Simultaneous Bidirectional Parallel Link with Per-Pin Skew Compensation”, Evelina Yeung, Student Member, IEEE, and Mark A. Horowitz, Fellow, IEEE, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 35, NO. 11, NOVEMBER 2000, pp. 1619 and “Architecture and Design of a Simultaneously Bidirectional Single-ended High Speed Chip-to-Chip Interface”, Robert Drost, SMLI TR-2002-107 February 2002.
Accordingly, one method to handle unwanted reflected signals on data connections is echo-cancellation. In FIG. 1, the signal driver 41 of unit A sends a pulse into the cable 10, but at the same time the same pulse is supplied to reference input pins of receiving comparator 31 by means of replica driver 51. The effect of the supplied pulse from the replica driver 51 to the receiving comparator 31 is illustrated for a single-ended signaling in FIG. 2 (left part of the Fig.).
In FIG. 2, by the signal from replica driver 51 the detecting level of the receiving comparator 31 is changed with an equal amount as the (send) pulse changes the signal at the output of signal driver 41. In this way, in principle, the receiving comparator 31 will not “see” the pulse sent by the signal driver 41 located at the same end of the cable 10. One remaining problem in such typical echo-cancellation system is matching of load coming from the cable 10 having connectors to the load seen by the replica driver 51. In particular, if the loads are not equal, the reference level of the receiving comparator 31 does not match to the signal sent into cable 10. Consequently, the difference (signal) is seen as signal by the receiving comparator 31 and thus is mixed with the intended signal sent from the opposite end of the cable, for example the unit B in FIG. 1.
Another even more difficult problem is formed by the signal reflected back from the other end of the cable from impedance discontinuities caused by, for instance, ESD protection circuits, connector capacitances and/or inductances. This reflected part was found as a serious problem, for example, in microwave link systems using simultaneous bidirectional signaling. Accordingly, there is a need for cable connections enabling high-speed serial simultaneous bidirectional communication.